In relatively recent times, applications have developed where it is economically or otherwise desirable to galvanize only one surface of a strip of steel. Other applications require coatings of different thickness on opposed strip surfaces.
Examples where a one-side coated steel is used are wall panels for buildings and automotive components. Automobile rocker panels, for example, frequently are heavily galvanized on their internal surfaces to inhibit corrosive attack by water trapped inside the panels, especially when that water contains road salt or other chemicals, while the external surfaces are provided with a smooth uncoated finish for appearance.
A differential coating is often desirable for automotive parts. A relatively thick coating of zinc is applied to interior surfaces of the part and a thin coating is applied to exterior surfaces. The thin exterior coating inhibits corrosion in the event of scraping and/or denting of the car finish.
While a need exists for steel which is galvanized on one surface, or alternately in a differential manner to both surfaces, the techniques which have been used in the past were wasteful and inefficient, or required an enormous investment, or both.
One technique for one side coating is to hot dip the steel in molten zinc with one face treated in a manner that is intended to prevent its being coated. Techniques for keeping the zinc on one side of a thin, flat strip product, however, have been difficult to achieve. The hot dip technique has also physically changed the properties of the steel being plated and does not produce the uniformity of coating which can be achieved with electroplating techniques.
A second known technique is to use a conventional electrolytic strip plating line modified to maintain the level of plating solution at a level where it contacts only the lower surface of a strip of steel being plated, in hopes of plating only that lower surface. Unfortunately, even when the level of the plating solution is controlled very precisely, there is considerable splashover and marginal portions of the top surface of the strip become plated due to this splashing of the solution. With this technique the top coated marginal portions are cropped off and only a central portion of a strip produces a useful one-side coated product. The cropped portions typically are scrapped or used in applications which require poor quality steel because, although perhaps plated effectively on the lower surface, the splashed over surface is irregularly and poorly plated and not useful for products demanding quality strip.
Other techniques for one side plating have been developed which mask one surface while plating the other. For example it is known to provide a strip of soft steel that is reeved over rollers that are partially immersed in a plating bath and function to mask the surface which they contact as the opposite surface is plated. It will be appreciated that the apparatus is complex and requires a very significant capital investment. The required capital investment is heightened when one appreciates that for automotive applications the galvanized coating must be relatively heavy which means either slow throughput, or alternately for an efficient line, a relatively long and expensive line to develop the thick coating desired.
Most known electroplating systems use consumable electrodes. That is, the electrodes each include a rather large piece of zinc for anodic solution to replenish the zinc ions plated out onto the workpiece. As electrode zinc is consumed, electrode-to-workpiece spacing changes and due to this and other variables, very precise and uniform plating thickness is difficult to achieve.
Because of the variables which are inherent in consumable electrode plating the equipment and controls for systems performing such plating are expensive and complex. For example, sophisticated electrical controls have been developed which monitor and compensate for variations in several plating parameters in an attempt to achieve more uniform plating with consumable electrodes.
There also have been proposals to use nonconsumable electrodes. A nonconsumable electrode is a conductive material which is maintained at a potential differential with the workpiece so that current flow between the electrode and the workpiece will plate zinc ions onto the workpiece from an electrically conductive plating solution filling the space between the electrode and the workpiece. As the ions are reduced to the metallic state onto the workpiece, however, the solution adjacent or near the workpiece becomes depleted of zinc metal ions. High speed efficient plating cannot therefore be achieved with nonconsumable electrodes unless the proper concentration of the zinc ions is maintained by other means at the workpiece surface. Problems of replenishing or maintaining the zinc ion concentration have inhibited the performance of prior nonconsumable anode systems, with the result that they have not enjoyed significant commercial success.
The use of a nonconsumable anode is shown in U.S. Pat. No. 2,244,423, to Hall. The anode disclosed in that patent includes a series of apertures through which plating solution flows to contact a strip to be plated. While in theory capable of achieving one side and/or differential two side plating the Hall structure is deficient for a number of reasons.
The Hall structure allows the plating solution to flow off the strip but this flow is constricted by gutters which bound the strip. This constricted fluid flow can cause the solutions's ion concentration near the strip to become depleted at an uncertain rate as plating occurs, with resultant non-uniformity of plating thickness.
A second deficiency of the Hall plating apparatus involves its orientation of anode and strip. With the anode mounted beneath the strip to plate the strip underside, it is possible that pockets of gas may collect on the strip as the plating process occurs. This problem is especially likely due to the gutter-caused constriction of fluid flow away from the strip. When a gas pocket forms, plating current from the anode to the strip is disrupted and non-uniform strip plating results.
A further problem inherent in the Hall structure is its use of multiple anodes across the workpiece which are separated by gaps. It is believed impossible to maintain such electrically isolated anodes at identical electrical potentials. Therefore, bi-polar plating action occurs between anodes. That is, a lower potential anode will act as a cathode to higher potential anodes and zinc will be plated onto the lower potential anode. Plating effectiveness of the plated anode is obviously reduced.
Use of separate anodes can result in non-uniform plating due also to non-uniform plating current density created by the gaps between anodes.